Gridiron football codes

American football is the most common and widely known of the gridiron-based football codes. It is played with eleven men to a side, four downs and a 100-yard field. The IFAF uses the name "American football" in its name and statutes, identifying it as being "made up by American football played under whatever set of rules, Canadian football, flag football, touch football, peewee-football, indoor American football and related activities for amateur and professionals". Note that the premier professional league in America, the NFL, has its own distinct code, Official Playing Rules of the National Football League. Colleges in America generally play under the code defined in NCAA Football Rules and Interpretations. High Schools in America generally follow the rules and interpretations published by the National Federation of High School Associations (NFHS), although at some states follow the NCAA code for high school play. Youth games (below high school age) generally follow NFHS code with modifications. Adult semi-pro, amateur, touch, flag, etc. may follow any one of these codes or use their own rules. While the vast majority of the game is the same among these three codes, subtle variations in rules can lead to large difference in play. Many of the differences are in penalty enforcement and the definitions of fouls.
Canadian football is played almost exclusively in Canada. It was originally more closely related to rugby until the Burnside rules were adopted. The game is played on a 110-yard field and has three downs and twelve men to a side. The Canadian game also allows players to move forward toward the line of scrimmage before the snap, which is forb dden in most versions of American football, and also features a one-point "single" (formally called a "rouge") for a ball kicked into the end zone and not returned by the receiving team. Like the American game, the Canadian Football League and Canadian Interuniversity Sport both have their own rulebooks for the game, although there are generally fewer differences than between their American counterparts.
Nine-man football, eight-man football and six-man football are varieties of gridiron football played with fewer players. They are played with four downs (often with a 15 yard requirement for a new set of downs, as opposed to 10 in other codes), fewer offensive linemen, and an 80-yard field. These games are generally based on the high school rulebooks, which have an addendum devoted to the play of these codes.
Indoor football is played with special rules to accommodate smaller indoor facilities. It is played on a 50-yard field with seven or eight men to a side (depending on the league). Prototype games were played in 1902 and 1932, both of which used the shortened field but followed the outdoor standard 11 men to a side. However, indoor football did not gain popularity until James F. Foster's proprietary version, arena football, debuted in 1986, and set most of the standards for indoor leagues today. As almost all indoor leagues are for-profit professional leagues, each league has its own proprietary code.
Touch football, flag football and backyard football are informal varieties of the game, played primarily at an amateur and recreational level. No specific rulebooks are universally recognized for these variants, where house rules usually apply.